fix a race condition in eglMakeCurrent()

it would happen when a context was made non-current, in this
case we would call the implementation's eglMakeCurrent() which
would succeed, if we're rescheduled at that point, another
eglMakeCurrent() could make that context current to another thread,
however, when we came back to the original call we would
overwrite egl_context_t internal state.

this is fixed by moving the critical section under
egl_display_t's lock.

Change-Id: I743c85696e13263d3a9570824940263df0caacdc